Explosives are used in open cut mines to facilitate either overburden removal or the mining of the final product. In a number of instances, the shock wave produced by a detonating explosive is too strong, resulting in overblasting and excess damage to the surrounding strata. This can lead to problems in final limits blasting, in blasting in weak or highly jointed material and in production blasting in soft rock. Cracks from blasting have been detected as far as 50 meters behind the borehole in the high wall and final limit areas of a mine situated in weak geological structures. Large pockets of fine material have also been observed after blasting in a coal seam. Both these situations indicated that the explosive used was too powerful for the material being mined. In particular, the peak shock energy of the explosive has been too great, resulting in the observed damage.
It is known to employ foams for sound insulation and attenuation purposes generally. One specific application is the absorption of shock waves from explosive devices such as car bombs and parcel bombs. The practice, as an alternative to disarming e.g., by a remotely controlled robot, is to envelop the bomb in foam at detonation. This technique has met with some success in reducing the air overblast produced by an explosive and thus reducing explosive damage to structures in the vicinity of parcel and car bombs. The tests carried out by the Defense Departments have been fairly rudimentary in nature, with few measurements being taken except for peak noise levels in the far field. Published papers have, e.g., examined the use of foams, especially acqueous foams, in the reduction of blast noise, demolition noise, gun blast noise levels, and the blast produced by a bomb in the trunk of a small car. The use of foam to provide blast attenuation in mining has not been canvassed.
A number of research programs have been conducted to ascertain the nature of the processes by which foams and other heterogeneous materials dissipate energy. Several such processes have been identified. Heat conduction losses result from the conduction of thermal energy between the liquid and bubbles and cause a phase difference between the pressure in the bubbles and the external pressure. Multiple reflections from bubble surfaces within the foam cause the energy to disperse. Energy dissipation also results from a viscous loss mechanism involving the friction of individual bubbles during oscillations, and exchanges of molecular energy that can lead to absorption of sound include the conversion of kinetic energy into stored protential energy (e.g., in structural re-arrangements), rotational/vibrational energy, or energies of association and dissociation. The multiple reflection mechanism has been found to become more prevalent as the foam becomes drier. The relative importance of each of these processes in the attenuation of sound, including shock waves, by foam has not been clearly established.